Single AD object synchronization to multiple Azure AD tenants and free/busy

Question

We have a single Active Directory domain and two email domains. Users are split into two Organization Units basis UPN/Email domain and synchronized through two Azure AD Connect servers to separate Azure AD tenants.

On the Exchange side, basis September 2020 update to Hybrid Configuration Wizard, we have configured a single on-premises two-tenant hybrid.

However, we have come across a challenge in how to make shared resources (meeting rooms, equipment, car parks) available to users in both tenants. Shared resources are still on premises.

Here is what we did:

1. We placed a couple of shared resource objects in a new OU and synchronized that OU through both Azure AD Connect instances.
2. We found that the object shows up in the address book of Exchange Online users in both tenants
3. However, the free busy information is only visible from one tenant
4. So, we added an alias/proxy/secondary address to the shared resource with verified domain of the tenant from where the free busy is not visible and voila it starts working

So, the following are my questions:

1. Is this a supported configuration for identity synchronization?
2. Is this a supported configuration for free/busy?
3. What are the possible challenges that may occur in future?

Answer 1

@Anonymous

One AD account can only supported to sync with one AAD.

I would suggest you migrate room mailbox to one of tenant, then do a cross-tenant free/busy sharing: September 2020 Hybrid Configuration Wizard Update

But, if your current configuration works, you can continue to use it, although it is not supported. You could modify it when you find issue with it.

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Answer 2

Syncing the same object to two separate tenants is not supported according to the supported topology documentation here:

https://learn.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-topologies

However, according to this same documentation page, it says you can use FIM 2010 or MIM 2016 on-premises to sync objects (via GALSync) between two Exchange organizations. The shared objects in one organization appear as foreign users/contacts in the other organization. These different on-premises Active Directory instances can then be synchronized with their own Azure AD tenants. @David Lundell or @David Lundell can confirm.